This invention is in the field of fluorinated carbon materials, in particular subfluorinated graphite and coke particles.
Fluorinated carbons are used commercially as a positive electrode material in primary lithium batteries. Fluorination of graphite allows intercalation of fluorine between the carbon layers. Li/CFx battery systems are known to be capable of delivery of up to 700 Wh/kg, 1000 Wh/l, at room temperature, and at a rate of C/100 (i.e., a battery current of a 1/100th that of the capacity of the battery per hour). (See, e.g., Bruce, G. Development of a CFx D Cell for Man Portable Applications. in Joint Service Power Expo. 2005; and Gabano, J. P., ed. Lithium Batteries, by M. Fukuda & T. Iijima. 1983, Academic Press: New York). Cathodes in these systems typically have carbon-fluoride stoichiometries typically ranging from CF1.05 to CF1.1. This cathode material, however, is known to be discharge rate limited, and currents lower than C/50 (battery current 1/50th that of the capacity of the battery divided by 1 hour) are often necessary to avoid cell polarization and large capacity loss. High electronic resistivity up to 1015 Ohm·cm of CFx is a potential cause of the observed discharge rate limitations, as there is a strong correlation between cathode thickness and performance; thicker cathodes tend to be more rate-limited. (See, e.g., V. N. Mittkin, J. Structural Chemistry, 2003, Vol. 44, 82-115, translated from Zhurnal Structunoi Khimii, 2003, Vol. 44, 99-138).
Other industrial applications of fluorinated carbons include use as solid lubricants or as reservoirs for very active molecular oxidizers such as BrF3 and ClF3.
In a lithium/CFx cell, the cell overall discharge reaction, first postulated by Wittingham (1975) Electrochem. Soc. 122:526, can be schematized by equation (1):(CFx)n+xnLinC+nxLiF  (1)
Thus, the theoretical specific discharge capacity Qth, expressed in mAh·g−1, is given by equation (2):
                                          Q            th                    ⁡                      (            x            )                          =                              x            ⁢                                                  ⁢            F                                3.6            ⁢                          (                              12                +                                  19                  ⁢                  x                                            )                                                          (        2        )            where F is the Faraday constant and 3.6 is a unit conversion constant.
The theoretical capacity of (CFx)n materials with different stoichiometry is therefore as follows:
x=0.25, Qth=400 mAh·g−1; x=0.33, Qth=484 mAh·g−1; x=0.50, Qth=623 mAh·g−1; x=0.66, Qth=721 mAh·g−1; and x=1.00, Qth=865 mAh·g−1.
The reactivity of carbon allotropic forms with fluorine gas differs largely owing either to the degree of graphitization or to the type of the carbon material (Hamwi A. et al.; J. Phys. Chem. Solids, 1996, 57(6-8), 677-688). In general, the higher the graphitization degree, the higher the reaction temperature. Carbon fluorides have been obtained by direct fluorination in the presence of fluorine or mixtures of fluorine and an inert gas. When graphite is used as the starting material, no significant fluorination is observed below 300° C. From 350 to 640° C., two graphite fluorides, mainly differing in crystal structure and composition are formed: poly(dicarbon monofluoride) (C2F)n and poly(carbon monofluoride) (CF)n (Nakajima T.; Watanabe N. Graphite fluorides and Carbon-Fluorine compounds, 1991, CRC Press, Boston; Kita Y.; Watanabe N.; Fujii Y.; J. Am. Chem. Soc., 1979, 101, 3832). In both compounds the carbon atoms take the sp3 hybridization with associated distortion of the carbon hexagons from planar to ‘chair-like’ or ‘boat-like’ configuration. Poly(dicarbon monofluoride) is obtained at ˜350° C. and has a characteristic structure, where two adjacent fluorine layers are separated by two carbon layers bonded by strongly covalent C—C bonding along the c-axis of the hexagonal lattice (stage 2). On the other hand, poly(carbon monofluoride) which is achieved at ˜600° C. has a structure with only one carbon layer between two adjacent fluorine layers (stage 1). Graphite fluorides obtained between 350 and 600° C. have an intermediary composition between (C2F)n and (CF)n and consist of a mixture of these two phases (Kita, 1979, ibid.). The stage s denotes the number of layers of carbon separating two successive layers of fluorine. Thus a compound of stage 1 has a sequence of stacking of the layers as FCF/FCF . . . , and a compound of stage 2 has the sequence FCCF/FCCF . . . . Both poly(dicarbon monofluoride) and poly(carbon monofluoride) are known to have relatively poor electrical conductivity.
U.S. Pat. No. 3,536,532 to Watanabe et al. describes a primary cell including a positive electrode having as the principal active material a crystalline fluorinated carbon represented by the formula (CFx)n. where x is not smaller than 0.5 but not larger than 1. As described, the carbon is fluorinated by heating the reactor to the desired temperature then introducing fluorine. U.S. Pat. No. 3,700,502 to Watanabe et al. describes a battery including a positive electrode having as its active material an amorphous or partially amorphous solid fluoridated carbon represented by the Formula (CFx)n, wherein x is in the range of from greater than 0 to 1. U.S. Pat. No. 4,247,608 to Watanabe et al. describes an electrolytic cell including a positive electrode having as the main active material a poly-dicarbon monofluoride represented by the formula (C2F)n wherein n is an integer.
Lam and Yazami (Lam, P. et al., 2006, J. Power Sources, 153, 354-359) present results for sub-fluorinated graphite fluorides (CFx)n where 0.33<x<0.63.
Carbon-fluorine intercalation compounds have been also obtained by incorporating other compounds capable of acting as a fluorination catalyst, such as HF or other fluorides, into the gas mixture. These methods can allow fluorination at lower temperatures. These methods have also allowed intercalation compounds other than (C2F)n and (CF)n to be prepared (N. Watanabe et al., “Graphite Fluorides”, Elsevier, Amsterdam, 1988, pp 240-246). These intercalation compounds prepared in the presence of HF or of a metal fluoride have an ionic character when the fluorine content is very low (F/C<0.1), or an iono-covalent character for higher fluorine contents (0.2<F/C<0.5). In any case, the bonding energy measured by Electron Spectroscopy for Chemical Analysis (ESCA) gives a value less than 687 eV for the most important peak of the F1s line and a value less than 285 eV for that of the C1s line (T. Nakajima, Fluorine-carbon and Fluoride-carbon, Chemistry, Physics and Applications, Marcel Dekker 1995 p. 13).